Wavelength modulation spectroscopy (WMS) has long been the preferred technique for high-accuracy remote path-integrated gas concentration measurements due to its high-sensitivity and inherent immunity to many sources of measurement noise and bias. (See, e.g., Bomse, D. S., et. al., “Frequency modulation and wavelength modulation spectroscopies: comparison of experimental methods using a lead-salt diode laser,” Appl. Opt., 31, 718-731 (1992); and Iseki, T., et. al., “A Compact Remote Methane Sensor using a Tunable Diode Laser,” Meas. Sci. Technol., 11, 594 (2000).) Due to these benefits there are now several commercially available hand-held sensors based on WMS for remote methane leak detection (See, e.g., the Remote Methane Leak Detector by Heath Associates. US; and a hand-held sensor by Tokyo Gas Co.).
These sensors perform well in leak detection scenarios involving short standoff distances and in relatively small search areas. However, their relatively long measurement times (0.1 s), lack of spatial registration of individual measurements, and lack of distance information to the backscattering target preclude imagery generation, large area scanning, and quantitative concentration analysis.
WMS sensors also exist for longer-range remote methane sensing from helicopters for pipeline leak monitoring. (See, e.g., the Aerial Laser Methane Assessment (ALMA) System offered by Pergam Technical. Services.) This type of sensor suffers from many of the same limitations as the hand-held devices. Its slow measurement acquisition time (0.04 s) precludes spatial scanning of the WMS beam and results in a data product consisting of a single line of measurements roughly positioned around the pipeline. The line measurement format precludes many desired data products including leak localization, quantitative estimates of total leaked gas and gas flux estimates.
Many emerging gas detection applications will benefit from rapidly-acquired, accurate, quantitative, and long range gas concentration imagery covering large measurement areas. Examples include emissions monitoring of methane, CO2 and other hazardous gases from large industrial facilities to comply with new air pollution standards set forth by the EPA, pipeline leak detection and monitoring, and environmental terrestrial monitoring to understand large-scale sources and sinks of greenhouse gases and how they contribute to climate change.
The disclosed systems and methods herein teach how to create accurate and precise path-integrated gas concentration imagery of a scene from a collection of spatially-scanned and -encoded WMS measurements. It also teaches how the path-integrated gas concentration imagery can be converted into path-averaged gas concentration imagery with the addition of spatially encoded distance measurements to objects in the scene. It is shown that path-averaged gas concentration imagery may be superior to path-integrated gas concentration imagery for applications requiring high-sensitivity detection of regions containing elevated (or otherwise anomalous) gas concentration. Also, methods are presented for rapid measurement processing via a simplified representation of the WMS signal model to permit timely generation of gas concentration imagery with reduced systematic measurement errors.